1. Field of the Invention
The present invention relates to a method for making masks using lasers and in particular to exposure masks and punch inspection masks made using lasers.
2. Description of Related Art
Masks are substrates having a defined pattern thereon and are used in a number of industrial applications in a wide variety of fields. The electronics field uses masks extensively to make printed circuit boards and integrated circuits and the different masks employed are generally categorized as exposure or photo masks, screening masks and inspection masks.
For making integrated circuits (IC) on a silicon chip, the initial design and layout of an IC are carried out on a scale several hundred times larger than the final desired dimensions of the circuit pattern. The initial layout of an IC is normally done on a magnification scale in a range of 250:1 to 500:1 and the layout is a composite of different mask patterns corresponding to the different masking steps associated with the fabrication process. Each mask pattern is cut into a dimensionally stable plastic laminate layer, called a Rubylith, which consists of a clear Mylar base with a pealable opaque ruby overlay. The overlay can be cut with a sharp knife and removed to form clear areas in the opaque overlay. Photographic techniques are then used to reduce each of these base layers to the final circuit dimensions. This is a time consuming, labor intensive and costly process.
During the masking operation, the wafer surface to be masked is coated with a photosensitive coating known as a photoresist or resist. The exposure mask is then placed on the surface of the wafer and exposed with ultraviolet light. Depending upon whether a negative-active resist or a positive-active resist is used, a pattern is formed on the photoresist layer on the wafer by the ultraviolet light and the exposed wafer is developed in a number of steps with liquid solvents to form the desired pattern on the wafer. A similar process is used to form circuit patterns on printed circuit boards using an exposure mask, exposure to ultraviolet light and developing of the exposed photoresist layer.
In other electronic applications such as in the manufacturing of multi-layered ceramic (MLC) packaging for semiconductor electronic circuits, green (unfired) ceramic sheets are formed by a well known doctor blade process. These greensheets are typically punched by mechanical means using various methods as well as by laser and Electron-beam equipment. The resulting holes (VIAS) are subsequently filled with conductive materials and the greensheets patterned to produce a desired electrical conductive path. The individual greensheets are precisely stacked one upon another, and laminated under high pressure to form a green laminate. The laminate is then cut to the desired size and sintered in a kiln to form the finished ceramic substrate ready for further processes if desired.
It is extremely important that the punched vias are accurately placed in the greensheet so that the vias align precisely to the screening mask so that the metal paste fills them completely and that screened patterns do not short to the nonaligned vias. To ensure that the vias are properly placed, a punch inspection mask is typically used. These inspection masks are employed to visually verify that the punch data used to punch the sheets is correct and also to debug the punch device for certain products as well as ensuring that the correct shrinkage has been applied when creating the data sets.
These masks are now fabricated using process steps similar to those used to fabricate thick film screening masks using a photolithographic process where art work is generated on masters and subsequent photo expose/develop processes performed to make the mask. This process is expensive and it requires a relatively long time to make each mask. A typical process for making masks is shown in U.S. Pat. No. 3,720,143.
Research Disclosure, September 1986, No. 269, shows high resolution molybdenum screening masks made using a laser etching process. The thin molybdenum masks usually 2 mils thick are used as templates through which metal paste is screened onto ceramic green sheets to form conductive lines after lamination and sintering. The article discusses forming these type masks using conventional wet lithographic processes and the masks limitations in being able to form lines only as small as 3 mils wide. Using laser etching however, features as small as 10 microns can be etched through the 2 mil molybdenum sheets and typical results for 1-2 mil wide slots can be etched with a separation as little as 3 mils. The laser burns the molybdenum metal in air and oxidizes the molybdenum to MoO.sub.3 which is volatile at the elevated process temperatures. Recrystallized MoO.sub.3 debris can be removed mechanically or by dissolution in a variety of solvents.
Laser processes have been used extensively for marking materials with letters, numbers, special symbols and company logos. A "Neodymium yttrium aluminum garnet (Nd:YAG) laser marking system" is shown in SPIE Vol. 247, Advances in Laser Engineering and Applications (1980) pages 18-23. The laser marking system is controlled by a microprocessor and is composed of three fundamental components: 1) a laser that emits pulses of radiation on command, 2) an optical train to direct and focus that radiation to a spot on the workpiece and 3) electronic circuitry to control the optical system and fire the laser at appropriate times. The electronic circuitry is organized around an 8085 microprocessor and allows the operator to directly enter the information to be marked. Marking is done on a variety of substrates such as plastics by heating the plastic so it decomposes and changes color providing a high contrast mark. Marking is also shown on transparent materials using a proprietary process which produces highly visible masks on transparent materials. It is cautioned that the use of high laser energy can damage the transparent material and cause localized in homogeneities which result in sporadic, inconsistent marks. Marking of painted surfaces is also disclosed to provide a contrast with the host material substrate.
Laser marking is shown in U.S. Pat. No. 4,578,329 wherein a laser radiation absorbing substance is incorporated into a plastic to absorb the laser beam and decompose the plastic forming the desired marks. Japanese No. 1224186 shows a mask for laser marking having a patterned metal layer on a polyolefin layer and radiating a laser beam onto the surface to be marked through the mask.
U.S. Pat. No. 4,340,654 shows a process for repairing transparent defects in photomasks by applying a coating material which absorbs radiant energy over the transparent defect and fusing the material to the substrate with a laser forming an opaque layer and eliminating the transparent defect in the mask.
Through-holes are shown formed in a plastic sheet in Japanese 515987 by using a metal foil mask over the sheet and drilling the through-holes using a laser beam by repetitive irradiation through the mask openings.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a laser method for making masks, and in particular inspection masks and exposure masks, useful in a number of applications especially electronic applications, which method is inexpensive and cost effective.
It is another object of the present invention to provide inexpensive and cost effective masks, in particular inspection masks and exposure masks, made by a laser process which may be used for a variety of electrical applications.
A further object of the invention is to provide inexpensive and cost effective inspection masks having a special pattern thereon made by a laser process to enable the user of the mask to easily and effectively determine if a cavity such as a via or a line in a substrate is within specification for size and/or position (centrality) on the substrate being inspected.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.